Communication Aided
Protection Schemes
Pilot Relay Schemes
Tribhuwan Choubey – SCE/TDBU/ECQ
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Introduction
• Relays at the Transmission terminals talk with
each other using telecommunication media.
• Pilot relays do not pick up for faults outside their
defined zone of protection.
• Pilot relays pick up instantaneously for faults
within their defined zone of protection.
• For increased reliability some type of backup
protection is also provided
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2- Zone Distance Relay
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Pilot relay telecommunication
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Relay communication
• Leased Telephone Line ( 56 or 64kbps over copper)
• Power Line Carrier (PLC – 64kbps) – Twisted copper
wire for 2/ 3 terminals
• Microwave or UHF radio (Multiplexed – 1.544 mbits/
sec DS1 signal – 24 channels)
• Dedicated Optical Fiber (Single Mode – 10 miles or
Multi mode – 60 miles)
• SONET (Multiplexed Symmetrical Optical Network Synchronous)
• T1 Multiplexing – 1.544 mbits/ sec (Asynchronous –
voice and data communication)
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Microwave Tower
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Microwave Tower
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Microwave Telecom Equipment
• Microwave Radio frequency spectrum: 1850-19909,
2130-2200, 6525-6875 and 12200-12700 MHz
• Operation: Two frequency duplex operation
• Telecom Equipment: Transmitters, Receivers, Order
wires, Filters, Power Supplies, switching devices
• Microwave RF Apparatus specifications: T-8505
• Carrier Frequency and Bandwidth : FCC Rules 94.65
and 94.71
• Frequency Stability: FCC rules 94.67
• Baseband I/O level: < -35dBm/ -15dBm per channel
• Power Supply: 48V DC
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Microwave Telecom Equipment
• Each terminal is equipped with hot standby transmitter.
• Switching circuitry is bilateral in between transmitters
• The input to the receivers from the antenna is connected
through an RF Hybrid
• Switching time for manual operation < 20 msec.
• Switching time for automatic operation < 25 msec.
• Operation of either or both of these switches cause a
“Guard” or “Off Normal” lamps
• Hot standby operation of Transmitters and Receivers
allows uninterrupted operation on failure of one
equipment
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SONETcommunication
SONET - Synchronous Optical Network
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SONET Data Service
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Fiber optic communication
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Fiber optic Data Losses
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T1 Extended Super Frame
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Categories of Pilot Relaying
Following categories of protection schemes
are typically applied in pilot relaying.
• Phase comparison
• Directional comparison blocking
• Remote tripping
• Transfer trip
• Line differential
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Pilot Relaying Philosophy
500 KV Pilot Relaying Schemes:
• Redundant Pilot relaying systems
• Redundant Transfer Trip relaying systems
• One set of electromechanical relays as
backup protection system.
• Mix of communication system like Power
Line Carrier (PLC) and Microwave are
used for communication redundancy
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Pilot Relaying Philosophy
220 KV Pilot Relaying Schemes:
• One Pilot relaying systems
• One set of electromechanical relays or
another Pilot relaying scheme with
microwave as communication channel are
used as backup protection system
• Mix of communication system like Power
Line Carrier (PLC) or Microwave are used
for communication redundancy
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Pilot Relaying Philosophy
• Principle of Differential Relaying is applied for
instantaneous tripping for in zone faults
• Four wires, normally termed as Pilot Wires, run
in between substations to compare current
between terminals (Typical F/C 15ms – 25ms)
• HCB Relays from Westinghouse (ABB) are
predominantly used as Pilot relays for lines up to
20 miles.
• HCB relay contains Sequence filters, DC polar
unit with Operating and Restraining coils,
Insulating and Saturating Transformers
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Differential Relaying Philosophy
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Pilot Relaying Philosophy
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Pilot Relaying Philosophy
• Westinghouse (ABB) LCB relays are used for
Pilot Relaying for Transmission lines from 20 to
250 miles length.
• The communication medium is Fiber Optics
• The relay contains Sequence Filter, Transmitter
Modulator ( 1 Ph -> FM), Receiver De Modulator
(FM –> 1Ph), Phase comparison Unit (compare
currents at Local/ Remote terminal)
• For out of zone fault, the signals would be out of
phase and in phase for fault in the zone.
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Pilot Relaying Philosophy
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Phase Comparison Relaying
• Relay combination used with phase comparison systems are SLD or
SKBU carrier relaying and CO( Residual Ground)/ ICC (Directional
ground).
• Phase comparison relaying compares current phasors at local and
remote terminals. Line and Neutral potential are not required.
• Faults in the protected zone would cause the currents to trip in less
than 3 cycles
• Fault detector turns on carrier signal during Fault
• Transmitter generates blocking pulse during negative half cycle.
• During external fault both transmitters produce additive blocking
pulse for the full cycle.
• During internal fault out of phase pulse cancel blocking pulse,
allowing the relay to trip.
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Phase Comparison Relaying
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SPCU relays are used for Phase comparison for Phase and Phase to
Ground faults
Microwave is used for communication of current phasors. Guard signals
are transmitted for communication health. Relay trips in 9 cycles on loss
of guard signal. Noise on communication channel will block tripping.
SRU/ UIO as auxiliary relay for annunciation purpose
Directional comparison blocking ( SLYP/ SLYN) compares current
direction at both terminals to determine in zone or out of zone faults to
block tripping
SLYP provides Zone1 element ( 80% of line: instantaneous trip with
TT), Zone2 (125% of Line) (Instantaneous for in zone fault and TT or
blocking signal for out of zone faults) and Zone3 (180% of line sends
blocking signal out of zone faults)
SLYN provides Directional overcurrent, trip for in zone faults and block
for out of zone faults
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DCB Philosophy
• Impedance relays trip instantaneously for z1 faults
covering 80% of line
• Over reaching impedance relays with z2 faults operate
with time delay covering 125% of the line
• Directional comparison Blocking (DCB) allows over
reaching relays to trip instantaneously by blocking out of
zone faults
• Thus Phase Distance Relays and ground over current
relays can provide instantaneous protection for in zone
faults
• Only one Communication channel is used for sending
blocking signal only , used with PLC schemes
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DCB Relaying Philosophy
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Typical Transfer Trip Relaying
• Transfer Trip schemes are another flavor of Directional comparison
schemes, where two relays communicate to trip rather than block.
• The two frequencies transmitted through two communication
channels are:
– Guard frequency to keep guard relays closed to block tripping over
channel noise
– Tripping Frequency to allow tripping relay to pickup and trip circuit
breakers
• Basic Transfer trip Schemes:
– Direct Transfer Trip (DTP)
• Direct Overreaching Transfer Trip (DOTT) – 125%
• Direct Under reaching Transfer Trip (DUTT) – 80%
– Permissive Transfer Trip
• Permissive Overreaching Transfer Trip (POTT) – 125 %
• Permissive Under reaching Transfer Trip (PUTT) –80%
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Typical PUTT Relaying Scheme
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Typical PUTT Relaying Scheme would have following components:
– 3 GCX (Directional Distance Relays ( Z1, Z2 and Z3 protection)
– Z2 and Z3 timer provides backup delayed tripping to POTT
– 1 JBCG directional Ground Relays
– 2 Transmitter and Receiver systems
– 2 Transfer Trip communication channels
– 1 Ground Fault detector relay (SC)
– 1 HD Balance relay to be used as backup for Transfer Tripping
– Trip signal at Remote terminal is enabled only when both transmitters
are keyed to trip simultaneously, if Local and Remote permissive trip
signal is available
– Loss of Guard signal for six cycles continuously will disable the channel
equipment and if it continues for more than 2 seconds resulting into
alarm and if alarm is continuous, then the relays are made inactive
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PUTT Relaying Philosophy
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Typical POTT Relaying Scheme
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Typical POTT Relaying Scheme would have following components:
– 3 CEY (Impedance relay with zone 2 timer- starts at fault detection)
– 1 SLYG directional Ground Relays
– 2 GE Type 40 Transmitter and Receiver systems
– 2 Transfer Trip communication channels (Primary/ Secondary)
– When a terminal receives the shift from Guard to trip frequency and a
trip signal is present from Local relay, the local CB will trip
– Zone 2 timer is reset after the fault is cleared
– If zone 2 timer times out it will trip the local CBs irrespective of Tone
signals from receiver to provide a backup protection
– If the fault is out of zone, then relay at the remote terminal will not
pickup and removes the Guard frequency, preventing local relay to trip
– Loss of Guard signal for six cycles continuously will disable the channel
equipment and continuous alarm will deactivate the relays
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POTT Relaying Philosophy
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Digital POTT Relaying Scheme
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Some of the new microprocessor based relays being used in the industry are as
follows:
D60 Hybrid POTT Relays from GE
L90 Differential with DTT from GE
PLC Transfer Trip via RFL-9780
MW Transfer Trip via RFL-9745
FO Transfer Trip via RFL-9745
SEL 421 – POTT/ DTT with PLC from Schweitzer Electric
SEL-311L – Differential with DTT from Schweitzer Electric
SEL-311C – POTT from Schweitzer Electric
Communication Details:
– Serial RS485 communication Port
– Ethernet full duplex network
– Modbus communication Protocol
– IEC61850 communication data architecture
– Communication speed 9600MB/sec
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Issues in Relay Communication
• Optical fiber cable damage or too high output from
communications interface causes loss of channel signal
alarm and may need attenuation or proper splice tray
connection.
• T1 communication standard (DS1) requires transmittal of
at least one pulse within any eight bit sequence. Loss of
synchronization and unlocked frequency clocks could
result in frame slips. Resulting in data loss.
• Leased Phone Lines need CSU channels as
demarcation point between vendor and customer’s
communication terminals. Interference on the cable/
CSU may cause loss of signal.
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Issues in Relay Communication
• High induced voltages across pilot wire possibly caused
by faults or lightning could cause induced fluxes and rise
in ground voltage. This is remediated through provision
of neutralizing reactors, voltage limiting circuits or surge
suppressors.
• Open or shorted pilot wires are monitored and resulting
erroneous trip signals are blocked.
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